A process for making porous track membranes is known in which a polymeric film is irradiated by heavy charged particles and chemically etched with a hexavalent chromium solution at a high temperature; see Russian Federation Patent No. 2056151, IPC B 01 D 67/00, Bulletin No. 8 of 20 Mar. 1996. This prior art process has many disadvantages. The chromium compounds are firmly adsorbed on the surface of polymeric film that results in making it considerably difficult to remove these compounds during the washing step. The subsequent presence of these compounds results in fouling of the filtered fluid when the membranes are used in the separation of materials from fluids.
Another prior art process for making porous track membranes comprises etching a tracked fluoropolymeric film with an etching solution containing potassium permanganate and sodium hydroxide at temperatures of 80-110° C. in a closed vessel; Shirokova V. V., and Tretyakova S. P., “Physical And Chemical Basis For Manufacturing Of Fluoropolymer Track Membranes”, Radiation Measurements, Vol. 28, Nos. 1-6, 1997, pages 791-798. The disadvantages of this prior art process include: (1) the considerable increase in etching time by operating below 100° C. and the resulting poor efficiency and the non-uniform heating by not circulating the etching solution, and (2) the resulting variability of the etching rates at different points on the film surface.
There is a need for a more efficient process for producing higher quality porous track membranes.